The CAN bus system is used in the context of communication between sensors and control devices, for example in automobiles. In the CAN bus system, messages are transmitted by way of the CAN protocol as described in the CAN specification in ISO 11898.
In the CAN bus system, the bit rate is the same in all portions of the protocol. The maximum bit rate is 1 Mbit/s, i.e., the bit time tbit is 1 μs. In a further development of the CAN protocol, namely CAN with flexible data rate (CAN FD), in contrast to the classic CAN protocol, at the end of the arbitration phase the data rate or bit rate for the subsequent data phase is raised to, for example, 2 Mbit/s or 5 Mbit/s. This entails correspondingly shorter bit times tbit, for example tbit=200 ns for a data rate or bit rate of 5 Mbit/s. This is described more precisely in the current ISO standard 11898-1 (under development) or the specification entitled “CAN with Flexible Data Rate, Specification Version 1.0 (released Apr. 17, 2012)” constituting a CAN protocol specification with CAN FD.
According to CAN Physical Layer Standard ISO 11898-2/-5/-6, a CAN bus system is to be constructed in such a way that at least two subscriber stations or nodes, such as sensors or control devices, etc., are each connected to a bus line via a stub line. The bus line is ideally terminated with a respective terminating resistor at the two ends of the bus line. This topology ideally exhibits no transients upon a change in bus state from dominant to recessive or vice versa. According to CAN Physical Layer Standard ISO 11898-2/-5/-6, only this topology is to be used.
What is being observed nowadays, however, that in reality so-called star topologies, having only one terminating resistor, are instead being used more and more often. This is advantageous principally in the manufacture of vehicles, since it simplifies the manufacturing process and interim inspections. This topology and termination has the bad property, however, of impressing strong dynamics in the form of oscillations on the bus lines when transceiver output stages switch off, especially upon transition of the bus signal from dominant to recessive. In the worst case, the oscillations do not decay over the entire bit time tbit of a signal bit, and are then, as a result of the conditions described below, undesirably detected as oscillations at the terminal for the received signal RX of the receiving subscriber station.
The nominal bit time N is subdivided into four phases: a Sync_Seg(N) phase, a Prop_Seg(N) phase, a Phase-Seg1(N) phase, and a Phase-Seg2(N) phase. The Sync_Seg(N) phase encompasses 1/N of the nominal bit time N, the Sync_Seg(N) phase encompasses 5/N of the nominal bit time N, and the Phase-Seg1(N) phase and Phase-Seg2(N) phase each encompass 4/N of the nominal bit time N.
At the receiving subscriber station, the bit is sampled at a definable point in time within the nominal bit time N. This definable point in time is also called the “sample point.” The sample point generally is programmed between Phase_Seg1 and Phase_Seg2. The corresponding new bus state is recognized only if it is present at the receiving node at the time of the sample point.
Long-lasting oscillations in bus voltage due to technically incorrect termination and topology of the bus systems are consequently a factor that, in the present-day real conditions as described above, complicate or prevent error-free data reception at the receiving subscriber station. In the context of CAN FD, a further factor that complicates or prevents error-free data reception at the receiving subscriber station is a shortening of the bit time tbit due to an increasing bit rate.